1. Field of the Invention
The present invention relates to thermoforming generally and, more particularly, but not by way of limitation, to a novel web clamp for use in thermoforming processes.
2. Background Art
Thermoforming is widely used for producing three dimensional objects from relatively thin sheets of thermoplastic materials, such objects often being used a containers or parts of containers for goods, and are typified by blister pack covers, pastry domes, and fast-food sandwich containers, to name only a few examples.
In a typical thermoforming process, a rectangular mold base is provided which may have one or more cavities defined therein or may have provision made for affixing thereto one or more mold cavities. A web of thermoplastic material is fed from a roll of such material, heated and then indexed over the mold cavities. A plurality of holes defined through the walls of the cavities is connected to a vacuum source to draw the thermoplastic material into the cavities. To assist this drawing operation, pressure is applied to the upper surface of the thermoplastic material. Such pressure is supplied from a rectangular pressure box which is temporarily placed over the mold base and removably sealed thereto, the pressure box having an open base and a top with four walls depending therefrom. After forming, coolant may be supplied to the mold base and the molded objects are removed from the mold base typically by means of a stripper plate built into the mold base.
Movement of the web is achieved by means of pin chains disposed at either side of the web, with pins inserted into edges of the web, and with the chains moving along a defined path. When the pressure box is sealed to the mold base, the pin chains secure the web against sidewise movement during the molding operation. To secure the web against lengthwise movement in the index direction, clamps are provided at either end of the mold base, with a clamp bar of each clamp assembly firmly clamping the web to the front and rear edges of the pressure box as the pressure box and the mold base engage each other. Each clamp bar is conventionally moved by vertical shafts rising from two vertical air cylinders to press the web against grooves formed in the edges of the pressure box. A disadvantage of such conventional web clamps is that the vertical shafts are rigidly attached to the clamp bar and are unable to accommodate any mechanical misalignments or distortion of the web. As a result, the vertical shafts are subject to damage. A further disadvantage of the conventional arrangement is that the air cylinders used have relatively large diameters. This requires that the mold be disposed farther from the heater than is otherwise desirable, in order to accommodate the diameter of the air cylinders.
Accordingly, it is a principal object of the present invention to provide a web clamp for thermoforming operations that can accommodate mechanical misalignments or distortion of the web.
It is a further object of the invention to provide such a web clamp that is economically constructed.
It is an additional object of the invention to provide such a web clamp that reduces the required spacing of thermoforming mold and heater.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.